1. Field of the Invention
The present invention relates to a digital signal processing system and method applied for chroma transition, and more particularly to a system that selectively mixes previous, present and delayed chroma signals to generate an optimized chroma signal to improve color sharpness in a television.
2. Description of Related Art
In usual video signal (NTSC or PAL standard) processing, a video signal is separated into picture information and non-picture information. A chrominance (chroma) signal and a luminance signal which comprise the picture information are then subsequently extracted. Based on the chroma signal, two orthogonal factors can be further derived from the chroma signal. The chroma signal together with the two orthogonal factors are usually referred to as Y, U, V signals (or Y, I, Q signals or Y, Cb, Cr signals). The chroma signal is carried over the luminance signal, wherein the phase of the chroma signal can represent a unique color. During chroma transition processes, the phase of the chroma signal will accordingly be changed. With the increase in the phase of the chroma signal, the time of the chroma transition will be extended, which detracts from color sharpness.
Picture signals including those of the NTSC, PAL and SECAM type all can be represented by combinations of the chroma and the luminance signal. The chroma signal bandwidth is narrow in comparison with the luminance signal bandwidth. Because of the limited frequency bandwidth of the chroma signal, chroma signal transitions (transients) are relatively slow. That is to say, the slope of a transition representing color edges has only a moderate slope, which when displayed for viewing detracts from sharp color demarcations.
Some chroma transition approaches, which are directly performed on the chroma signal, have been proposed to improve the color sharpness. For example, with reference to FIG. 6, U.S. Pat. No. 5,920,357 issued to Ohara entitled “Digital color transient improvement” mainly uses two time delay circuits (61)(62), a band pass filter (63) and a median filter (64) to improve the chroma transition. With reference to FIG. 7, another similar approach is disclosed in U.S. Pat. No. 5,936,682 entitled “Circuit for enhancing chrominance transitions in real-time video reception” that utilizes multiple time delay circuits (71), a band pass filter 72 and a comparison circuit (73) to improve the chroma transition.
The two prior arts are indeed able to enhance the color sharpness. However the implementation of the digital filter is rather complex and expensive. Further, the output waveform of the median filter (64) is not ideal.
Another known technique is shown in FIG. 8, U.S. Pat. No. 6,008,862 entitled “Perceived color transient improvement”, which adjusts the luminance signal based on the detected status of the chroma signal without directly modifying the chroma signal. The chroma signal (Cin) and the luminance signal (Yin) are respectively input to two edge detectors (81)(82). A multiplier (not numbered) multiplies the logical signals from the edge detectors (81)(82) and furnishes a logical control signal to an artificial peaked signal generator (83). Preferably, the edge detector (81) furnishes edge parameters like width and steepness of the edge to the artificial peaked signal generator (83). An output signal from the generator (83) is applied to an adder (not numbered), which also receives a peaked luminance signal from an optional luminance peaking circuit (84) to which the input luminance signal Yin is applied. The adder supplies the output luminance signal Yout. The output chroma signal (Cout) is identical to the input chroma signal (Cin).
The architecture of FIG. 8 is mainly applied to the color display apparatus that receives video signals composed of three primary color signals, red, blue and green. The three primary color signals can be converted into Y, Cb and Cr signals through linear converting functions. Therefore, if the luminance signal has any change, the chroma signal is basically supposed to be affected. However, as shown in FIG. 9, the output chroma signal (Cout) is identical to the input chroma signal (Cin). That means there is no improvement in performance of the chroma signal.
With reference to FIG. 10, U.S. Pat. No. 6,571,224 entitled “Fuzzy logic based color transition improvement method and system” adopts the fuzzy theory to optimize color transition. The system uses two parameter tables to control the color transition, wherein the first table is established based on first-order difference signals and second-order difference signals, and the second table is established based on the time when each of the first-order signals takes place. Even though the system does not need any filters, these fuzzy logic blocks (91)(92), calculation blocks (93)(94) and signal differential circuits are still complex.